A conventional Claus process consists of a thermal unit and a catalytic unit. The thermal unit consists of a reaction furnace, a waste heat boiler, and a sulfur condenser. In the thermal unit hydrogen sulfide and oxygen react to produce elemental sulfur and sulfur dioxide. The elemental sulfur is removed in the sulfur condenser. The catalytic unit includes a series of catalytic reactors, where each catalytic reactor is preceded by a re-heater and followed by a condenser. In the catalytic reactors elemental sulfur is produced by the reaction among hydrogen sulfide, sulfur dioxide, and oxygen. The re-heater increases the temperature to the reaction temperature in the catalytic reactors. The condenser condenses the elemental sulfur to produce a liquid sulfur stream. The re-heater is a direct-fired reheater that burns fuel gas to increase the process stream temperature.